Universal joints are used in a wide range of applications to transmit rotary motion between an input and output shafts which may not be coaxial.
One such application is in aircraft to transmit power from a centralised power drive unit to a plurality of actuators that are located along leading and trailing edges of wings. Depending upon the location of the power transmission line with respect to the wing neutral axis, the transmission line can see length changes as a function of wing bending in flight manoeuvres and in high load phases of normal take-off and landing cycles. This, together with the torque loads which the joints are intended to accommodate, can develop axial loads within the transmission system that must be reacted by suitable aircraft structure. These loads are a function of the spline size that connects the universal joints with associated actuators or shafts, and can develop in the order of 9 kN of axial load. Substantial aircraft structure must be provided to counter such forces. It would therefore be desirable to reduce the axial forces experienced in a transmission line and reduce the forces acting upon actuators, joints and supporting structure.